1. Field of the Invention
The present invention relates to a thin film transistor (TFT) and a method for fabricating the same.
2. Discussion of the Related Art
Flat panel displays are noted because of their light weight, thin structure and low power consumption. Examples of flat panel displays having higher image quality include a liquid crystal display (LCD) and an organic light emitting display (OLED).
The flat panel displays may use a thin film transistor (TFT) as a switching device. A semiconductor layer of the TFT comprises amorphous silicon or polycrystalline silicon.
In comparison to amorphous silicon, polycrystalline silicon has superior field effect mobility, response time and stability of temperature and light. The TFT including the semiconductor layer composed of polycrystalline silicon has an advantage in that it is possible to form the TFT and the driving integrated circuit (IC) on the same substrate. There is no need for the TFT to be connected to the driving IC.
The semiconductor layer composed of the polycrystalline silicon may be formed by depositing the polycrystalline silicon or by crystallizing an amorphous silicon layer after deposition. There are many methods of crystallization, for example, Solid Phase Crystallization (SPC), Metal Induced Crystallization (MIC), Eximer Laser Annealing (ELA) and Sequential Lateral Solidification (SLS), and the like.
ELA is a representative method to crystallize the amorphous silicon. Crystallization by ELA is performed by irradiating the amorphous silicon layer with a laser beam, such as XeCl laser, having a wavelength of about 308 nm. Use of ELA is very common because crystallization by ELA results in lower thermal impact on the glass substrate and a polycrystalline silicon layer crystallized by ELA has superior properties as compared to a polycrystalline silicon layer crystallized by other methods.
FIG. 1A is a cross-sectional schematic view showing a polycrystalline silicon layer crystallized by ELA, and FIG. 1B is a photograph of the surface of a polycrystalline silicon layer crystallized by ELA.
Referring to FIGS. 1A and 1B, the polycrystalline silicon layer 120 has protrusions C formed in the grain boundary region B between adjacent grains A. The protrusions C are generated because of a difference in the density of adjacent grains that occurs when the amorphous silicon is rapidly crystallized after being melted.
The polycrystalline silicon layer 120 has inferior surface uniformity and a gate insulating layer subsequently formed with the polycrystalline silicon layer 120 will also have inferior surface uniformity. The inferior surface uniformity causes leakage current in the TFT and hillocks in the metal layer subsequently formed overlying the gate insulating layer on the gate electrode. Moreover, the protrusions C cause incorrect exposure and etching in the photolithography process used to pattern the metal layers, thereby degrading the properties of TFT and decreasing its reliability.